1. Field of the Invention
The present invention relates to a battery device (hereinafter referred to as "battery pack") including a circuit for which the monitoring of a battery state such as a voltage across a secondary battery or a charging or discharging current is required, a battery state monitoring circuit, an external connection terminal to the exterior of the battery device, a switching element, the secondary battery and a sense resistor.
Also, the present invention relates to a battery state monitoring circuit and a battery device which have a high accuracy in detecting an over-current and controls which the discharge current by detection of a voltage between both ends of the sense resistor.
2. Description of the Related Art
As a conventional battery state monitoring circuit, there has been known a device shown in a circuit block diagram of FIG. 2. The structure of this type is disclosed, for example, in Japanese Patent Unexamined Publication No Hei 9-312172, "A battery pack, a charger and a charging system as well as a charging method". This publication relates to a battery pack which is called a "smart battery system" or the like. In other words, this is a battery pack having a function of monitoring a voltage across a secondary battery or a charging or discharging current by use of a microcomputer and which conducts communication with a load such as the charger or the computer.
The use of the battery pack thus structured makes it possible to recognize the battery state through communication with a charger, a microcomputer within a personal computer, a load or the like. By use of this information, the display of a remaining capacity of the battery, the termination of charging and discharging and so on are accurately conducted.
FIG. 2 shows a conventional battery state monitoring circuit 18A and a conventional battery device which is made up of at least an external terminal made up of a plus terminal 1A and a minus terminal 2A which connect the battery state monitoring circuit 18A, a charger and so on, and a switching element 11A that controls a discharge current. In the conventional battery state monitoring circuit 18A and the conventional battery device, a voltage Vd between both ends of the switching element 11A (for example, MOS-FET) is detected by a microcomputer to turn off the switching element 11A so that charging batteries 6A to 8A are prevented from allowing an excessive current to flow. Reference 3A denotes an operational amplifier; 5A, a microcomputer for calculating the remaining capacity of the charging batteries 6A to 8A through communication with the battery state monitoring circuit 18A; 16A, a load connected to the battery device; 17A, a charger connected to the battery device; 19A, a cut-out circuit for cutting out charging/discharging current paths of the charging batteries 6A to 8A by the battery state monitoring circuit 18A; and 20A, a selecting circuit for determining which battery is selected among the charging batteries 6A to 8A.
The conventional battery state monitoring circuit 18A and the conventional battery device suffer from a problem in that accuracy of over-current detection is low because the on-resistance (a resistor during energization) of the switching element 11A (for example, a MOS-FET) has a fluctuation in manufacture, and a (gate) voltage dependency, a temperature characteristic and so on are low.
According to the present invention, there is provided a battery state monitoring circuit and a battery device for detecting an over-current by detecting a voltage between both ends of a sense resistor. Further, provision of means for holding an over-current detection signal allows prevention of the oscillation of a switching element.